1. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a method and apparatus for providing power control in a closed-loop communication system.
2. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users exist. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation techniques of CDMA has significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d assigned to the assignee of the present invention, and is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d also assigned to the assignee of the present invention, and is incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user or mobile station through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention and incorporated by reference herein.
If the transmission power of signals transmitted by the base station to a mobile station are too high, it can create problems such as interfering with other mobile stations. As a result, most base stations have a fixed amount of power at which to transmit signals, and therefore can transmit to only a limited number of mobile stations. Alternatively, if the transmission power of signals transmitted by the base station are too low, then the mobile station can receive multiple erroneous transmitted frames. Terrestrial channel fading and other known factors can affect the transmission power of signals transmitted by the base station. As a result, each base station must rapidly and accurately adjust the transmission power for the signals which it transmits to the mobile stations. A method and apparatus for controlling transmission power is disclosed in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
The CDMA communication system can also exchange signals with fixed location stations. Such fixed stations typically employ directional antennas and thereby communicate with only certain sectors of one or more base stations. Fixed stations also typically lack fading concerns caused by movement of the station. Consequently, the fixed nature of such fixed stations, their directional antennas, as well as other factors, allow such stations to enjoy increased performance over mobile stations. Such fixed stations often operate as wireless local loops with their corresponding base stations, and thus require less overhead from the communication system. As a result, subscribers of fixed stations typically enjoy less expensive service.
The present invention allows a base station or base station controller to select a method of controlling transmission power to and from a remote station based on a determination as to whether the remote station is fixed or mobile. For example, the base station can determine the status of a subscriber based on, for example, an electronic serial number in a registration signal received therefrom. If the electronic serial number received corresponds to a fixed station, then the base station lowers one or more power thresholds. Based on such lowered thresholds, the base station thereafter transmits forward link signals to the fixed station at a lower power level. As a result, the base station can increase its capacity due to the reduced power transmission to fixed remote stations. Similary, the remote station can adjust its thresholds based on whether it is being operated in a fixed or mobile mode. If the base station determines that the fixed station has moved from its prescribed location, then the base station can adjust its thresholds accordingly or restrict communication of the fixed station.
In the exemplary embodiment, the transmission power of the station is controlled by comparing its received Signal to Noise Ratio (SNR) at the base station against a threshold, then transmitting a power control command to increase or decrease remote station power based upon that comparison. The SNR threshold is driven by the Frame Error Rate (FER) of frames received at the remote station. The transmission power of signals transmitted by the base station to the remote station is controlled in accordance the forward link (FL) frame error rate which is reported by the remote station. The present invention is equally applicable to both of these control methods.
In a broad sense, the present invention embodies a communication system having a base station, at least one fixed location user station and one mobile user station. System users of the user stations exchange signals with the base station. A method for controlling transmission signal power of transmitted communication signals for the base station includes the steps of: (a) determining if a currently received signal was transmitted by the fixed user station, and (b) adjusting transmission of signals to the fixed user station if the current signal was transmitted by the fixed user station.